This invention relates to bone fusion devices. More specifically, it relates to devices that fuse spinal vertebrae together.
Fusion cages provide a space for inserting a bone graft between adjacent portions of bone. In time, the bone and bone graft grow together through or around the fusion cage to fuse the graft and the bone solidly together. One current use of fusion cages is to treat a variety of spinal disorders, including degenerative disc diseases such as Grade I or II spondylolistheses of the lumbar spine. Spinal fusion cages (included in the general term, xe2x80x9cfusion cagesxe2x80x9d) are inserted into the intervertebral disc space between two vertebrae for fusing them together. They distract (or expand) a collapsed disc space between two vertebrae to stabilize the vertebrae by preventing them from moving relative to each other.
The typical fusion cage is cylindrical, hollow, and threaded. Alternatively, some known fusion cages are unthreaded or made in tapered, elliptical, or rectangular shapes. Known fusion cages are constructed from a variety of materials including titanium alloys, porous tantalum, other metals, allograft bone, or ceramic material.
Fusion cages may be used to connect any adjacent portions of bone, however one primary use is in the lumbar spine. Fusion cages can also be used in the cervical or thoracic spine. Fusion cages can be inserted in the lumbar spine using an anterior, posterior, or lateral approach. Insertion is usually accomplished through a traditional open operation, but a laparoscopic or percutaneous insertion technique can also be used.
With any of the approaches, threaded fusion cages are inserted by first opening the disc space between two vertebrae of the lumbar spine using a wedge or other device on a first side of the vertebrae. Next, a tapered plug is hammered in to hold the disc space open. A threaded opening is then drilled and tapped on a second side opposite the first side of the vertebrae for producing the equivalent of a xe2x80x9csplitxe2x80x9d threaded bore defined by the walls of the vertebrae above and below the bore. The threaded fusion cage is then threaded into the bore and the wedge is removed. The first side is then drilled and tapped before inserting a second threaded fusion cage. Typically, two threaded fusion cages are used at each invertebral disc level.
There are problems with all of the standard approaches. With a posterior approach, neural structures in the spinal canal and foramen need to be properly retracted before the plug is hammered into the disc space. Proper neural retraction is critical to the insertion process. If the retraction is not done properly, the procedure could cause neural injury, i.e., nerve damage and potential neurologic deficit. With either the anterior or lateral approach, blood vessels or other vital structures need to be retracted and protected to reduce or eliminate internal bleeding.
The general technique for inserting fusion cages is well known. Insertion techniques and additional details on the design of fusion cages is described in Internal Fixation and Fusion of the Lumbar Spine Using Threaded Interbody Cages, by Curtis A. Dickman, M.D., published in BNI Quarterly, Volume 13, Number 3, 1997, which is hereby incorporated by reference.
U.S. Pat. No. 5,782,832 to Larsen et al. (the xe2x80x9cLarsen referencexe2x80x9d) discloses an alternate type of spinal fusion implant. FIG. 1 of the Larsen reference shows an implant apparatus with two separable support components which are adapted for adjusting sliding movement relative to each other to selectively vary the overall width of the implant to accommodate vertebral columns of various sizes or to vary the supporting capacity of the implant during healing. Each of the support components include upper and lower plate portions that are operatively connected by respective linkage mechanisms. The linkage mechanisms allow relative movement of the upper and lower plate portions between an extended position and a collapsed position. The device disclosed in the Larsen reference has several problems. One problem is that, because the width of the implant is adjusted prior to insertion, a wide insertion slot is necessary despite the reduced profile presented by the collapsed implant. Another problem is that at least part of the linkage mechanism extends beyond the upper and lower plate portions, thus requiring more invasion into the body cavity to position the implant. Yet another problem is that the linkage mechanisms must be locked into the expanded position by conventional arrangements such as locking screws.
The problems discussed above in regard to known fusion cages are substantially solved by the present invention.
The present invention is directed to a fusion cage that can be inserted less intrusively and requires a reduced size opening for insertion than known fusion cages. Reducing the size of the opening reduces and perhaps eliminates the need for retraction of neural structures, vascular structures, or other vital structures. Consequently, compared to known fusion cages, the fusion cage of the present reduces operating time, reduces blood loss, and reduces the risk of injury. It is believed that the present invention provides these and other advantages.
One preferred embodiment of the interbody fusion cage of the present invention includes an upper body and a lower body connected by articulated supports. The articulated supports enable the fusion cage to be collapsed prior to its insertion between adjacent vertebrae. Once inserted, the articulated supports allow the fusion cage to be expanded to a fully expanded position.
In another preferred embodiment, the fusion cage includes protrusions on the articulated supports, or ridges or other surface irregularities along the fusion cage""s upper and lower surfaces, to secure the fusion cage in position.
In an alternate preferred embodiment, an overcenter latch mechanism may be incorporated to maintain the fusion cage in the fully expanded position. Buttressing, or stops, located where the articulated supports attach to the upper or lower body, prevents the articulated supports from continuing out past the desired maximum height. Once at the maximum height, the forces exerted on the fusion cage by the bone surfaces above and below it continue to force the articulated supports outward against the stops.
Bone, or other material intended to promote bone growth, can be inserted into the cavity formed by the upper and lower body and the fully extended articulated supports. Eventually, adjacent vertebrae will grow through and around the fusion cage, effectively fusing the two vertebrae together.